The selection advantages associated with advanced biological maturation vary according to playing position in national-level youth soccer

This study investigated the extent to which biological maturation selection biases existed according to playing position in national-level youth soccer. A total of 159 players from the U13 to U16 age groups in the Football Association of Ireland’s national talent pathway and international representative squads had their relative biological maturity status assessed using the Khamis-Roche method for the percentage of predicted adult height at the time of observation. Players were categorised as goalkeeper (GK), central defender (CD), full-back (FB), centre defensive midfielder (CDM), centre midfielder (CM), centre attacking midfielder (CAM), wide midfielder (WM) or centre forward (CF). A series of one-sampled means t-tests were used to examine the degree to which biological maturation selection biases existed across playing positions. A non-parametric Kruskal-Wallis test was used to evaluate inter-positional differences. A small to very large selection bias in favour of early maturing players existed for GK (D = 0.7), CD (D = 1.65), FB (D = 0.49), CM (D = 0.62), WM (D = 0.78), and CF (D = 0.76) (p < 0.05). Maturational selection biases did not exist for CDM or CAM. Moreover, CD were significantly more advanced in maturation compared to FB, CDM and CAM (p < 0.05). This study supports the contention that maturation selection biases exist in youth soccer, but the magnitude of this bias is highly dependent upon playing position. The very strong maturity selection biases at the national level evidenced in this investigation highlight the need for Football Associations to explore strategies, such as futures programmes, to help to retain talented, yet late maturing athletes.

INTRODUCTION excluded). In contrast, using the same positional categorisations, other authors have found differences in skeletal maturation between regional-level youth players to be negligible [22]. Regardless, these positional categorisations fail to recognise the distinct physical, technical and tactical variations between players of the same broad playing position (e.g., full-backs vs. central defenders, central defensive midfielders vs. central attacking midfielders). More research is required

Biological maturity status
The maturity status of each player was estimated using the percentage of predicted adult height [23,24]. Among children of the same chronological age, it is assumed that those closer to their predicted adult height are more advanced in maturation compared to those further removed from their predicted adult height. The Khamis-Roche method enables the prediction of a player's adult height using the regression formula based upon age and gender-specific regression and early maturing boys have been shown to constitute as much as 74% of players within some academy cohorts [16]. As a direct consequence, late maturing boys are underrepresented within professional football academies, and in some instances, are deselected by age 14-15 years [15]. Indeed, early maturing boys are up to twenty times more likely to be retained within the academy system [16].
From a developmental perspective, many equally talented yet late maturing boys are subsequently denied exposure to the professional coaching, sports science and medical support, superior training equipment and facilities, and the high levels of competitive challenge that are experienced by those within the academy system [15,16].
Once excluded, these late maturing players are less likely to be represented at the professional level [15].
Although the selection of youth players more advanced in maturation may elicit some immediate advantages (i.e., competitive success), it can be counterproductive in regard to long-term player development. The physical, physiological and functional advantages associated with advanced biological maturation are attenuated or even reversed by adulthood [4]. Consequently, by excluding later maturing players, football clubs and associations have excluded a large proportion of the available pool of talent to select from, in turn reducing the number of high potential players that can be produced for the senior level of the game. Of interest, several scholars have proposed that later maturing players may hold the greatest potential for senior success [4,18]. The reasoning behind this proposition is attributed to the late maturing players being forced to overcome their biological limits through the development of other skills necessary for successful performance, including superior technical skills and tactical understanding, as well as the utilisation of more adaptive engagement in self-regulated learning, particularly self-evaluation and learning [4,18]. The high-pressured academy environment may also encourage early maturing boys to utilise their athletic advantages in order to be successful at the expense of their technical-tactical and psychological development [4]. Moreover, early maturing players are likely underchallenged when competing against less mature players who are not as physically able, which may further hinder the development of the technical-tactical and psychological skills necessary to succeed at the highest levels [19,20]. Malina et al. [21] investigated variations in skeletal maturation in national-level youth footballers by playing position and found that forwards were the most advanced, followed by defenders and then midfielders. However, the participant sample was small (n = 17) and aged only between 15-16 years, and players were broadly categorised as either defender, midfielder or forward (goalkeepers Biology of Sport, Vol. 40 No3, 2023 717 Biological maturation in youth soccer coefficients detailed by Khamis and Roche in their analysis of residents enrolled in the Fels longitudinal study [23,24]. The Khamis-Roche protocol requires the current age, height and weight of the child, and biological mid-parent height (mean height of biological parents).
Players had their body height measured to the closest 0.1 cm using a stadiometer (SECA, 217, Vogel and Halke, Hamburg, Germany) and their body mass measured to the closest 0.1 kg using digital scales (SECA, 877, Vogel and Halke, Hamburg, Germany). Parents' heights were self-reported and then adjusted for overestimation as outlined by Epstein et al. [25].
Mean adjusted paternal and maternal heights (178 cm and 165 cm, respectively) were in line with sex-specific means for Irish adults [26]. In instances where a biological parent was not in contact with a player and their parent/guardians, a national average for adult height was used for that biological parent (this accounted for approximately 8% of the sample). The median error bounds between actual and predicted adult height using the Khamis-Roche method is 2.2 cm in males aged between 4 to 17.5 years. For the age groups examined in this study, 12 to 16 years, the lowest 50% error was 1.3 cm for 16-year-olds, and the highest 50% error was 2.8 cm for 14-year-olds [23,24]. Using the percentage of predicted adult height has demonstrated construct validity as an indicator of biological maturation status in samples of healthy American, Portuguese and German youth [17,27,28].
The height of each player was expressed as a percentage of predicted adult height which was used as an estimate of biological maturity status at the time of observation [29]. Estimated relative biological maturity status was then expressed as a Z-score using the child's percentage of adult height compared to age-specific means and standard deviations outlined by Bayer and Bailey in the Berkeley Growth Longitudinal Study [30]. Comparisons to reference values outlined by Bayer and Bailey have been utilised in recent football-specific studies on other European players [7,15,17,19,31]. These Z-scores were then used to classify the youth players as late, on-time or early maturing. A Z-score of -0.5 to + 0.5 was classified as ontime maturity status; a Z-score of > + 0.5 was classified as early maturity status; and a Z-score of < -0.5 was classified as late maturity status (as currently employed in the English Premier League Player Management Application; [15]). The ± 0.5 criteria were selected over the traditional ± 1 criteria to better differentiate between players that were more advanced or delayed. Using the latter criteria, players presenting maturity Z-scores of -.99 and + .99 would be both considered 'on-time', when in reality there is almost 2 standard deviations of variance in maturation between them. All growth and maturation assessments and calculations were made by the first author who was trained using standardised field practices.

Playing position
Players were categorised as either goalkeepers (n = 17), full-backs

Data Analysis
Data were analysed using SPSS Version 27. Descriptive statistics were used to examine the variance in biological maturation status  defenders was also not conducted [33].
At the youth level, players advanced in maturation perform more high-intensity actions (running ≥ 1 s at > 19 km · h -1 ), repeated highintensity actions, and attain faster peak speeds during matchplay [13]. In addition, early maturing boys are generally taller and have greater muscle mass and strength [8] which is beneficial during physical challenges. It is thus unsurprising that the maturation biases are more pronounced in the outfield positions in which many of these physical characteristics are most desirable. Central defenders were the most biologically advanced in this sample. Indeed, defenders were also amongst the most mature players in similar samples [21,33]. This is likely due to the technical-tactical positional requirements in which greater size/stature and strength (e.g., to compete in aerial duals) is highly desirable in this position. During elitelevel match-play, sprinting is the most frequent action in goal-scoring situations [34]. As advanced maturation is associated with increased levels of testosterone and maturation of the anaerobic system [3,35], this may explain why the centre forwards and wide .

RESULTS
The results of the one-sample mean t-tests to examine the presence of maturity selection biases by playing position are presented in Table 1. The mean value for relative maturation was significantly greater than the expected value for goalkeepers, central defenders, full-backs, centre midfielders, wide midfielders, and centre forwards the onset of puberty and increases with chronological age and the level of competition [15,16]. Results from this study demonstrate that the magnitude of these selection biases is also heavily influenced by playing position. It has previously been suggested that maturation biases may be the most prevalent in the central positions [15], but these results do not directly support this hypothesis. Interestingly, wide midfielders and full-backs were generally early maturing and more biologically advanced than both central attacking and defensive midfielders.
During senior-level match-play, central attacking midfielders proportion of the available pool of talent to select from is reduced, and in turn, so is the potential to maximise the quality and quantity of players produced for the senior level of the game [16]. Given that all players mature by adulthood, and therefore senior levels of performance, the effects of advanced maturation will no longer be present by this point. Yet, the later maturing boys deselected from the system are unlikely to return in the future despite possibly holding potential for senior success [4,16,18]. Late maturing youth who are techno-tactically and psychologically skilled need to be nurtured through the system until biological maturity is attained [33]. However, talent identification is a difficult and complex process for coach- In immediate terms, several other strategies could be used to support the development of late maturing players, as well as players of on-time and early maturation statuses. One potential intervention is to reduce the pitch sizes during training/competition to emphasise the use of technical skills over physical/physiological attributes (and advantages) [41]. Similarly, matching players by maturity status during one-on-one training drills or matches (e.g., playing a right wide midfielder against a biologically matched left back) may help to support this process. In broader terms, the provision of generic coach education workshops and resources that educate clubs and their coaches in the area of growth and maturation may be a supplementary level of support that could be provided by Football Associations.
Ongoing educational support for practitioners at the club and nation-